[plt-scheme] MysterX bug squished

> It's amazing that this bug hadn't surfaced before.  That suggests that
> COM server implementors rarely use inheritance in their coclasses.

I think that's true.  The inheritance in this case was for versioning
reasons - newer methods were added to a new interface which inherited from
the old interface.  Other than that, inheritance for traditional subtyping
purposes is not really all that common in COM - mainly, I think, because
most use of COM is simply as an API-creation mechanism, that provides a
simplified view of the internal structure of an application, which is
designed to be as straightforward as possible.  Even in C++ programs with
complex class hierarchies and use of non-COM mixins, in most cases, any COM
interface provided is generally pretty flat.

Anton